46 Damage to Plant Tissue from Oapsid Buys 



(5) Suspension of Bacillus vulgaris. 



(6) Suspension as in (5) but boiled. 



(7) Suspension of Bacillus sublilis. 



(8) Suspension as in (7) but boiled. 



(9) Suspension of Bacillus mycoides. 

 (10) Suspension as in (9) but boiled. 



These inoculations were also made into willow leaves. In each case 

 only slight damage was caused, in no way comparable to that produced 

 by the Capsid. 



The foregoing experiments seem to prove conclusively that bacteria 

 play no part in producing the damage resulting from the feeding of the 

 bug. The fact that the insects from the moment of hatching produce 

 the same damage as the adults, also militates very strongly against the 

 theory of bacterial infection as it would necessitate the bacteria passing 

 on from generation to generation and also their having to pass the 

 winter in the egg. The chrome-red colonies of bacteria which were 

 isolated from the damaged apple leaf were presumably merely living on 

 the surface and had no connection with either insect or damage. 



There is left now the third possibility, i.e. that the salivary secre- 

 tions injected into the tissue have a toxic effect. There are known to be 

 two ducts (Awati(4)) down one of which there passes the salivary juices 

 while up the other passes the plant sap, presumably mixed with some 

 saliva. The saliva is injected under pressure by means of the very 

 powerful pump situated in the head of the insect. After the leaf has 

 been punctured and the insect's stylets withdrawn, a drop of fluid exudes 

 from the hole and slowly grows in size as the cells below are killed and 

 give up their contents. This is in marked contrast to the prick with a 

 sterile needle or the puncture made by the harmless bugs where no drop 

 of fluid exudes. The theory of harmful salivary injection is certainly 

 the most probable of the three possible explanations, and further ob- 

 servations seem to bear this out. 



The salivary glands of Capsids in general consist of a paired bilobed 

 gland (see Fig. 4) situated in the meta-thorax one on each side, from 

 the centre of this runs a long tube with apparently glandular walls, up 

 to the neck where it doubles back again, ending near the gland proper 

 in a very thin walled vesicle or reservoir which is not secretory ; a second 

 tube arises at the same point as the first and runs straight to the neck 

 where it connects with the pump. Attempts were made to discover if 

 any morphological difference existed between the salivary glands of 

 P. rugicollis and those of other apple-feeding Capsids. 



